Electronic sensors for detecting and evaluating materials such as liquids and gases found in the environment are known. Such an electronic sensor is sometimes called an “electronic nose” and is typically fabricated on a silicon substrate using conventional integrated circuit techniques at a relatively high cost. Electronic environmental sensors are widely useful in industrial systems and for measuring environmental contaminants.
A typical system includes a sensor portion that outputs electronic signals in response to an analyte. The electrical signals are filtered, amplified, and analyzed by a signal processor or other processing device or computer. The signal processing can be performed with computing circuits formed on the same substrate as the sensor. U.S. Pat. No. 5,337,018 illustrates an electronic sensor for determining alcohol content of fuels.
The sensor portion can include one or more spaced-apart electrodes in a variety of configurations. For example, U.S. Pat. No. 5,337,018 illustrates linear interdigitated electrodes and U.S. Pat. No. 7,520,173 illustrates electrodes formed in concentric circular or polygonal patterns. U.S. Pat. No. 6,730,212 discloses a sensor for chemical and biological materials that includes metal interdigitated electrodes coated with a hybrid polymer-base conducting film.
Electrode sensors are made using a variety of technologies including integrated circuit photolithographic methods, screen printing with thick films of silver and silver-palladium inks, electroplating to deposit a uniform layer of patterned copper, or by patterning sputtered or vaporized metal coating using laser ablation or photolithographic methods including liftoff and etching through a patterned mask layer. Photolithographic processes are known to be expensive, and generally require a rigid substrate for the formation of small feature size, e.g. <5 microns. Screen printing permits reliable formation of structures and patterns but only for a gap width or feature size of greater than 75 microns. Laser ablation or scribing uses a high-power excimer laser such as a Krypton-fluoride excimer laser having a wavelength of 248 nm to etch or scribe individual lines in the conductive surface metal coating to provide insulating gaps between residual conductive metal forming electrodes and other desired features. Laser ablation requires a time-consuming rastering technique if a complex electrode pattern is to be formed on the surface. Moreover, the precision of the electrode edge is not well defined. Sensor layers with embedded micro-channels are also known for pressure sensors.
Although electronic sensors are widely useful, the cost associated with the desired feature sizes can limit their applicability.